"For when you absolutely, positively, have had enough of freakin asset spam."
The main reason asset spam has often been at the forefront of the meta for the past couple of years, was because of the inherent tempo their horizontal tactics gave them. While there have always all kinds of tools to get money and cards, there never has been a way to get enough clicks to both set up AND check every remote. And because of this, runners lost a ton of tempo checking and getting rid of assets which consistently cost more to trash than rez. It was either that, or forfeit the econ game to the corp.
And now in comes Marathon across the finish line. An event which does not trash itself, allowing you to keep playing it. This was you have all the clicks you need. Corp spends 3 clicks to make 3 new remotes? Play this and start your turn with 3 less credits, but still at 4 clicks and a lot wiser about the board state.
This card becomes absolutely bonkers when combined with other support. Got 3 Aeneas Informants on the board? Every run now nets you a 2 credit profit, provided the remote did not contain a must-trash asset. If you have the turtle out you get a juicy counter too. Very situational (janky?) but If you play By Any Means and Paparazzi you even get to clear every remote the corp has for free, provided you can afford playing this card over and over.
Is this card universally useful? Definitely not. The greatest weakness of this card lies in that it is match-up specific. If you are facing Glacier this card is completely useless. Even against decks which rely on a couple of remotes, this card is nothing spectacular. Also, good thing they made this 5 influence, because Anarchs (cough) did not need this card the way Shapers did. And another slight caveat, if the corp manages to make the run unsuccessfull by blowing up the server, the card trashes itself as you need your run to succeed.
But yeah, don't let that cute artwork fool you, Marathon is an absolute beast of a card.